Immersion coating of strip material



00h24, 1967 P.' J. @oon IMMERSION COATING OF STRIP MATERIAL Filed May l,1964 United States Patent iitice W836i Patented Oct. 24, 1967 3,348,964IMMERSION COATING OF STRIP MATERIAL Paul J. Good, Springwater, N.Y.,assiguor, by mesne assignments, to Minnesota Mining and ManufacturingCompany, St. Paul, Minn., a corporation of Delaware Filed May 1, 1964,Ser. No. 364,140 8 Claims. (Cl. 117-69) This invention relates to theapplication of liquid coatings to webs of material, and is especiallyuseful in the field of photography, for the application of photographicemulsions and other layers (e.g., subbi'ng coatings, iilter layers,etc.) to a previously formed web which may be iilm base or which may bepaper or other suitable sheet material.

An object of the invention is the provision of a generally improved andmore satisfactory apparatus for applying coatings to webs.

Another object is the provision of a generally improved and moresatisfactory method for applying coatings to webs.

Still another object is the provision of a coating apparatus and acoating method having automatic control of the thickness of the appliedcoating in propo-rtion to the rate of feeding of the liquid coatingmaterial to the apparatus.

A further object is the provision of a coating apparatus and a coatingmethod in which coating liquid can be applied to a web while the web istraveling at .a relatively high rate of speed, and in which thethickness of the coating can be controlled with such accuracy that themethod and apparatus are particularly useful for producing photographiccoatings.

A still further object is the provision of a method and apparatusemploying dip coating techniques with all the attendant advantagesthereof, but so improved with respect to prior dip coating techniquesthat this technique may be used efiiciently for the production ofphotographic coatings.

These and other desirable objects may be attained in the mannerdisclosed as an illustrative embodiment of the invention in thefollowing description and in the accompanying drawings forming a parthereof, in which:

FIG. 1 is a schematic vertical section taken transversely through oneform of apparatus of the present invention; and

FIG. 2 is a similar View showing another form, wherein two layers ofcoating material are applied.

Referring now to FIG. 1, there is shown `a coating roll 11 rotating on ashaft 13. The web of material to be vcoated comes to the roll at 15 fromany suitable source of supply, goes around the lower part of the roll,and

,leaves the roll upwardly at 17, proceeding thence to a there is aninclined blade which forms a top or cover for the trough on this side ofthe roll 11. The edge of the blade 25 is parallel to the lroll andfairly close to it, but nevertheless spaced from the roll by a distanceseveral times the thickness of the coating to be applied. For example,if a coating having a thickness in the range of 0.001 of an inch is tobe applied to the web 15, 17, the edge of the blade 25 may be spacedfrom the web by a distance of 0.012 of `an inch or more.

It should be emphasized at this point that the blade 25 does notfunction as a doctor blade. In the coating art,

doctor blades are well known, and they control the thickness of anapplied coating by scraping off an excess thickness of coating, leavingonly the thickness which will pass between the web and the closelyadjacent edge of the doctor blade. It is the spacing of the edge of thedoctor blade from the web which determines the thickness of coating. Butthis is not the case with the blade 25 of the present invention. Theedge of the blade in the present case provides a surface against which ameniscus may form, between the edge of the blade and the adjacent web,but the spacing of the edge of the blade from the web does not determinethe thickness of the coating. As already mentioned in connection withthe above specific example, the spacing of the edge of the blade fromthe web may be several times the thickness of the coating which is to beapplied.

Below the bottom face of the blade 25 there is a wall 27, spaced fromand preferably parallel to the bottom face of the blade, providingbetween the wall and the blade a passageway 29 in the form of a slotleading to a vacuum conduit 31. The pan or trough 21 and the blade 25extend through the full width of the face of the coating roll 11, andthe slot Z9 and vacuum chamber 31 preferably also extend through thefull width of the roll. There are suitable end walls (not shown) whichclose the ends of the pan 21, slot 29, and vacuum chamber 31, the endwalls extending very close to the end faces of the roll 11, to providerelatively tight sealing at these points. The vacuum chamber 31 isconnected by a conduit 33 to any suitable source of partial vacuum, suchas a vacuum pump or an eductor.

The liquid which is to be coated onto the web is supplied to the pan ortrough 21 by any suitable inlet conduit such as shown schematically at35. The liquid is supplied at a controlled rate through a metering pump.

With this apparatus, after an initial period of operation sufficient toreach a state of equilibrium, continued operation results in theapplication of coating material at a thickness dependent upon the rateat which Athe coating liquid is fed into the system through the conduit35. When initially starting up, coating liquid is supplied to the pan ortrough 21 in sufficient quantity to keep the liquid level in the part 23of the pan at least as high as the bottom edge of the coating roll 11,and partial vacuum is applied to the chamber 31 to a sutlicient degreeto draw the liquid level up at least as high as the lower edge of themeniscus blade 25, and preferably a little higher. Only enough vacuum isneeded to overcome the hydrostatic head from the bottom of the coatingroll to a point slightly above the meniscus which will be formed at 37at the lower edge of the blade 25. The surface of the liquid, drawn upinto the slot 29 by the partial vacuum, will be at some such point asindicated at 39.

As the coating roll 11 rotates in a clockwise direction, feeding the web15, 17 into the coating pan and then out again, the outer face of theweb 15, 17 is thoroughly immersed in and coated by the liquid in thepan, so that as the web passes upwardly at 17 `a meniscus will be formedat 37 between the edge of the blade 25 `and the web 17, and a thincoating of liquid will remain on the face of the web as it passesupwardly beyond the meniscus. Initially, while the coating liquid isbeing introduced through the conduit 35 at a metered rate to give therequired thickness of coating on the area of the web which passesthrough the coating pan per unit of time, the coating will take placewith the maximum possible hydrostatic head, from the level of themeniscus 37, as indicated by the line 41, to the bottom edge of thecoating roll 11. This creates the maximum possible hydrostatic forceacting on the meniscus to tend to draw it back against the direction oftravel of the web, thereby reducing the quantity of coating liquid whichstays on the web beyond the meniscus. Thus initially the amount ofcoating applied to the web will be less than the intended amount. Butsince the amount of coating liquid removed from the pan by the web isless than the amount which is being supplied `to the pan through thesupply conduit 35, the level of the coating liquid will gradually riseat the incoming side 23 of the pan until it reaches some such level asindicated at 43. This will `reduce the hydrostatic head shownschematically at 45, so that there is now less of a backward or downwardpull on the meniscus 37, and a greater quantity of the liquid coatingmaterial will stay on the web 17 as it passes upwardly beyond themeniscus.

Once the point of equilibrium is reached, the `apparatus is selfmetering from this point onward, and the thickness of coating applied tothe web is automatically adjusted to the rate of inow of the coatingmaterial into the coating pan. Assuming, of course, that the rate oftravel of the web remains constant and that the viscosity of the coatingliquid also remains constant, it will be seen that if the web has atendency to pickup coating liquid faster than it is being fed into thepan through the conduit 35, this will' lower the level 43 of the liquidat the incoming side of the pan, thereby increasing the hydrostatic head45 between this level 43 and the meniscus 37, and the increased meniscustension on account of the increased hydrostatic head will tend to pullthe meniscus down with greater force, thereby holding back more of theliquid from traveling upwardly with the `web 17, reducing the thickness`of the coating until equilibrium is again established. Conversely, ifthe web attempted to take the coating material at a slower rate than therate at which it is supplied to the pan, the level of the free surface43 would rise, thereby reducing the hydrostatic head 45 and reducing themeniscus tension or downward pull on the meniscus 37, so that more ofthe liquid would stay on the web 17 as it passed upwardly beyond themeniscus. Thus the system is self metering, and coats the web at exactlythe same rate at which the coating material is introduced into thecoating pan. If it is desired to produce a thicker coating, the coatingliquid is supplied to the pan at a faster rate, and if it is desired toproduce a thinner coating, the rate of supply of coating liquid isreduced. This results in a very satisfactory control of coatingthickness, particularly suitable for use in the photographic coatingfield, such as for coating an emulsion, a subbing coat, a lter layer, ora protective layer; Relatively high coating speeds are possible whilestill maintaining the accurate self-metering control of thickness.

This system may .also be used where it is desired to apply more than onecoating to the web at the same time. Referring to FIG. 2, there is shownan arrangement for applying two coatings. The first coating is appliedin the same manner above described in connection with FIG. 1, and thecorresponding parts of the lapparatus are identilied by the samereference numerals used in FIG. 1. The first coating on the web 17,applied from the dip pan 21, is indicated at 51, and for the sake ofclarity it is` greatly exaggerated in thickness.

On top of thefsloping meniscus blade 25 there is a structure `53 havingan inlet conduit 55 for introducing the liquid which is to form thesecond coating, owng into a distribution chamber 57 which extendscrosswise, parallel to the axis of the coating roll 11, through the fullwidth which is to be coated. From the distribution channel 57, a smallslot 59 extends forwardly toward the coating roll, between the upperface of the meniscus bar 25 and the lower face of the structure 53. Thisslot 59 likewise extends the full width of the web to be coated. Theincoming liquid for the second coating may enter the distributionchamber 57 at only one point or a few spaced points, but the crosssectional size of this channel 57 insures adequate distribution of theliquid throughout the entire width which is to be coatedrThe liquid runsdown the slope or incline of the top surface of the meniscus bar 25,owing by gravity, as shown schematically at 61, andi meniscus tension orhydrostatic head differential de-` scribed in connection with the firstcoating. The thickness of the second coating depends upon the quantityof coating liquid which is allowed `to flow downwardly through the slot59 and onto the top of the meniscus blade at 61., since the moving webwill pick up all of the available liquid at this point.v Hence thethickness of the second.'

coating is controlled by controlling the `rate of input feed of thecoating `liquid through the conduit 55 into the distribution channel 57.The input is controlled, as usual, by a metering pump which accuratelymeters the input flow.

It is seen from the foregoing disclosure that the objects and purposesof the invention are well Ifulfilled. It is to be understood that theforegoing disclosure is given by way of illustrative example only,rather than by way of limitation, and that without departing from theinvention, the details may be varied within the scope of the appendedclaims.

What is claimed is:

1. The method of applying a liquid coating of uniform and controllablethickness to a moving web of sheet matcrial, which comprises the stepsof providing a bath of the coating liquid, moving a web through a pathin which one face of the web enters the bath and then exits from thebath in an upward direction, forming a meniscus of the coatingliquidagainst said face of the web at the point where said face of themoving web exits from the bath, and subjecting the meniscus to downwardforce of variable amount varying in accordance with the liquid level ofone portion of the bath, so arranged that when said liquid level drops,greater downward force is applied to the meniscus to reduce the quantityof coating liquid carried upwardly from the bath past the meniscus bythe upwardly moving web, and when said level rises, lesser downwardforce is applied to the meniscus to allow a greater quantity of coatingliquid to be carriedupwardly fron the bath past the meniscus by theupwardly moving we 2. The method as defined in claim 1, including thefurther step of `applying a second coating of a second liquid over thefirst coating, by owing a quantity of the second coating liquid onto thetop of the meniscus in position to make contact with the first coatingin overlying relation thereto as the web with the first coating emergesupwardly from the meniscus.

3. Coating apparatus, especially `for applying a photographie coating toa moving web of sheet material, comprising a pan for holding a supply ofcoating liquid, a web guide roller for guiding a moving web of sheetmaterial downwardly into the liquid in the pan on the entrance side ofthe rollerand then upwardly out of the l liquid on the exit side of theroller, walls substantially sealing the pan in an airtight manner on theexit side of the roller, said walls including a meniscus bar having anedge parallel to and slightly spaced from the web at the exit side ofthe roller, so that a liquid meniscus may form between the edge of saidbar and the web passing said edge, means for partially evacuating thespace within said walls sufficiently to hold the free level of coatingliquid on the exit side of said roller at an elevation slightly abovesaid edge of said meniscus bar and substantially above the free level ofcoating liquid on the entrance side of said roller, and means forintroducing coating liquid into said pan at a metered rate equal to theintended rate at which coating liquid is to be removed from s aid pan bythe upward exit motion of said web, the hydrostatic head resulting fromthe `dilference in elevation between the meniscus and the free level ofliquid on the entrance side of said roller causing a force at themeniscus which automatically meters the amount of liquid coatingremaining on the web as it passes the meniscus, to equate it to rate atwhich coating liquid is introduced into said pan.

4. Apparatus as defined in claim 3, in which said meniscus bar has a topsurface inclining downwardly toward the moving web which passes upwardlypast said bar.

5. Apparatus as defined in claim 3, `in which said meniscus bar has atop surface inclining downwardly toward the upwardly moving web, furtherincluding means for flowing a layer of a second coating liquiddownwardly over said top surface so that the layer of second coatingliquid flows onto the meniscus and is applied as a second coating layerover the iirst coating layer as the web with the iirst coating layerthereon passes upwardly from the meniscus.

6. Coating apparatus for applying a coating liquid to a moving web ofsheet material, comprising a pan for holding a supply of coating liquid,guide means for guiding a moving web of sheet material downwardly intothe liquid in the pan on the entrance side of the guide means andupwardly out of the liquid in the pan on the exit side of the guidemeans, means extending parallel to and slightly spaced from the web onthe exit side of the gu-ide means yand forming .a surface against whicha liquid meniscus may form between it and the moving web, means forenclosing the pan on the exit side of said guide means in asubstantially airtight manner while leaving liquid on the entrance sideof said guide means subject to atmospheric pressure and leaving thesurface of the meniscus also subject to atmospheric pressure, means forpartially evacuating said enclosing means to a suicient degree to keepthe free liquid level therein at a level at least as high as theelevation of said meniscus, and means for feeding coating liquid intosaid pan at a metered rate equal to the rate at which it is desired tohave the coating on the web remove coating liquid from the pan, wherebyany lowering of the level of liquid in the entrance side of the pan,caused by removal of liquid by the web at a rate faster than freshcoating liquid is fed into the pan,

will increase the hydrostatic head caused by the difference in elevationbetween the meniscus .and the liquid level in the entrance -side of thepan and thereby increase the force at the meniscus so that less liquidcoating will be allowed to pass the meniscus, thereby causing the rateat which coating liquid is carried out of the pan by the web to be equalto the rate at which fresh coating liquid is fed into the pan.

7. Coating apparatus for applying a liquid coating of uniform andcontrollable thickness to a moving web of sheet material, comprisingmeans for holding a bath of coating liquid, means for moving a webthrough a path in which one face of the web enters the bath and thenexists from the bath in an upward direction, means for forming ameniscus of the coating liquid against said face of the web at thelocation where said face of the moving web exits upwardly from the bath,and means for subjecting the meniscus to downward force of variableamount varying in accordance with the liquid level of one portion lofthe bath, said last named means being so arranged that when the liquidlevel of said portion of the bath drops, greater downward force isapplied to the meniscus to reduce the quantity of coating liquid carriedupwardly from the bath past the meniscus by the upW-ardly moving web,and when the liquid level of said portion of the bath rises, lesserdownward force is applied to the meniscus so as to allow a greaterquantity of coating liquid to be carried upwardly from the bath past themeniscus by the upwardly moving web.

S, Apparatus as defined in claim 7, further comprising means for flowinga layer of a second coating liquid onto the top of the meniscus inposition to make contact with the rst coating in overlying relationthereto as the web with the first coating emerges upwardly from themeniscus.

References Cited UNITED STATES PATENTS 2,257,373 9/1941 Fanselow 11S- 63X 2,681,294 6/1954 Beguin. 2,995,469 8/1961 Claire 118--63 X ALFRED L.LEAVITT, Primary Examiner. A. GOLIAN, Examiner.

1. THE METHOD OF APPLYING A LIQUID COATING OF UNIFORM AND CONTROLLABLETHICKNESS TO A MOVING WEB OF SHEET MATERIAL, WHICH COMPRISES THE STEPSOF PROVIDING A BATH OF THE COATING LIQUID, MOVING A WEB THROUGH A PATHIN WHICH ONE FACE OF THE WEB ENTERS THE BATH AND THEN EXITS FROM THEBATH IN AN UPWARD DIRECTION, FORMING A MENISCUS OF THE COATING LIQUIDAGAINST SAID FACE OF THE WEB AT THE POINT WHERE SAID FACE OF THE MOVINGWEB EXITS FROM THE BATH, AND SUBJECTING THE MENISCUS TO DOWNWARD FORCEOF VARIABLE AMOUNT VARYING IN ACCORDANCE WITH THE LIQUID LEVEL OF ONEPORTION OF THE BATH, SO ARRANGED THAT WHEN SAID